Worklet: Add step comments in MainThreadWorklet::addModule()

Spec: https://drafts.css-houdini.org/worklets/#dom-worklet-addmodule

BUG=627945

Review-Url: https://codereview.chromium.org/2851693002
Cr-Commit-Position: refs/heads/master@{#467999}

third_party/WebKit/Source/core/workers/MainThreadWorklet.cpp

@@ -29,16 +29,52 @@ ScriptPromise MainThreadWorklet::addModule(ScriptState* script_state,
                                            "This frame is already detached"));
   }
 
+  // Step 1: "Let promise be a new promise."
+  ScriptPromiseResolver* resolver = ScriptPromiseResolver::Create(script_state);
+  ScriptPromise promise = resolver->Promise();
+
+  // Step 2: "Let worklet be the current Worklet."
+  // |this| is the current Worklet.
+
+  // Step 3: "Let moduleURLRecord be the result of parsing the moduleURL
+  // argument relative to the relevant settings object of this."
   KURL module_url_record = GetExecutionContext()->CompleteURL(module_url);
+
+  // Step 4: "If moduleURLRecord is failure, then reject promise with a
+  // "SyntaxError" DOMException and return promise."
   if (!module_url_record.IsValid()) {
-    return ScriptPromise::RejectWithDOMException(
-        script_state,
-        DOMException::Create(kSyntaxError,
-                             "'" + module_url + "' is not a valid URL."));
+    resolver->Reject(DOMException::Create(
+        kSyntaxError, "'" + module_url + "' is not a valid URL."));
+    return promise;
   }
 
-  ScriptPromiseResolver* resolver = ScriptPromiseResolver::Create(script_state);
-  ScriptPromise promise = resolver->Promise();
+  // Step 5: "Return promise, and then continue running this algorithm in
+  // parallel."
+  // TODO(nhiroki): Make the following sequence async.
+
+  // Step 6: "Let credentialOptions be the credentials member of options."
+  // TODO(nhiroki): Implement credentialOptions (https://crbug.com/710837).
+
+  // Step 7: "Let outsideSettings be the relevant settings object of this."
+  // TODO(nhiroki): outsideSettings will be used for posting a task to the
+  // document's responsible event loop. We could use a task runner for the
+  // purpose.
+
+  // Step 8: "Let moduleResponsesMap be worklet's module responses map."
+  // TODO(nhiroki): Implement moduleResponsesMap (https://crbug.com/627945).
+
+  // Step 9: "Let workletGlobalScopeType be worklet's worklet global scope
+  // type."
+  // workletGlobalScopeType is encoded into the class name (e.g., PaintWorklet).
+
+  // Step 10: "If the worklet's WorkletGlobalScopes is empty, run the following
+  // steps:"
+  //   10.1: "Create a WorkletGlobalScope given workletGlobalScopeType,
+  //          moduleResponsesMap, and outsideSettings."
+  //   10.2: "Add the WorkletGlobalScope to worklet's WorkletGlobalScopes."
+  // "Depending on the type of worklet the user agent may create additional
+  // WorkletGlobalScopes at this time."
+  // TODO(nhiroki): Create WorkletGlobalScopes at this point.
 
   // Step 11: "Let pendingTaskStruct be a new pending tasks struct with counter
   // initialized to the length of worklet's WorkletGlobalScopes."